In recent years there has been a trend towards making granular detergents having a higher bulk density than before. Various techniques of making dense granular detergents, and of processing low density granular detergents in such a way that the bulk density is increased, have been described. One example of a suitable technique for making dense granular detergents is known as "agglomeration". This term describes any process in which small particles of the components are processed in such a way that they are built-up (or "agglomerated") to form suitable granular components.
The ideal detergent agglomerate should have a high bulk density and a high surfactant content and yet still have good solubility and dispersion properties. It should also be possible to use a manufacturing process which is both efficient and versatile.
Different approaches to these objectives have been made in the prior art.
U.S. Pat. No. 4,970,017, issued on Nov. 13, 1990, discloses a process for preparing a detergent composition containing silicate in a kneader. The resulting composition is a solid which is shaped into pellets and then passed through a grinding step to reduce the particle size to a suitable powder.
EPA402111, published on Dec. 12, 1990, describes a process for preparing a dough which comprises surfactant, followed by a granulation step. The dough may comprise a "deagglomerating" agent but the particular benefits of combining certain surfactants with silicate are not disclosed. The surfactant dough is prepared by a mixing process, but extrusion is preferably avoided. A process of this type is described in th prior art portion of claim 1.
EPA508543, published on Oct. 14, 1992, discloses a process for preparing a high active surfactant paste composition in an extruder. Although a number of possible chemical stucturants and surfactants are mentioned, there is no disclosure of the particularly efficient structuring by a combination of silicate and linear alkyl benzene sulphonate at elevated pressures.
Whilst the prior art suggests the use of silicate in various granulation processes, problems of defining a process which is economical on an industrial scale, and which provides granular detergents having high surfactant activity remain. It is an aim of the present invention to provide a process for converting a surfactant paste into a free-flowing granular detergent having high surfactant activity and good handling and performance properties. In order to achieve the particular benefits of the process of the invention a specific surfactant paste which comprises silicate and linear alkyl benzene sulphonate is provided, the paste having a particular liquid crystal structure which is highly desirable in the process of the present invention.
It has now been surprisingly found that it is advantageous to incorporate a water-soluble silicate salt into the surfactant paste which comprises linear alkyl benzene sulphonate. The combination of these two specific ingredients, under conditions of high pressure such as those which may be created behind a die plate of an extruder, results in a surprising improvement of the effectiveness of structuring of the paste. The structured paste which has been forced under pressure through the extruder die no longer has the properties of a viscous liquid, but rather has the properties of a deformable solid. This can then be finely dispersed and agglomerated with builder powders to give a free-flowing granular composition which have an activity of at least 35%, preferably at least 50%.